1. Field of the Invention
The present invention relates to an improvement of a radio-frequency tag communication device and a radio-frequency tag communication system, which are capable of radio communication with radio-frequency tags for writing and reading information on and from the radio-frequency tags.
2. Description of the Related Art
There is known an RFID (Radio-Frequency Identification) communication system wherein a radio-frequency tag communication device (interrogator) reads out information, in a non-contact fashion, from small-sized radio-frequency tags (transponders) on which desired information is written. In this RFID communication system, the radio-frequency tag communication device is capable of reading out the information from the radio-frequency tags, even where the radio-frequency tags are contaminated or located at positions invisible from the radio-frequency tag communication device. For this reason, the RFID communication system is expected to be used in various fields, such as management and inspection of articles of commodity.
The radio-frequency tag communication device indicated above usually has a limited maximum distance of communication of about several meters, and the single radio-frequency tag communication device is not capable of adequate data communication with radio-frequency tags disposed in a comparatively ample space. In view of this fact, there have been proposed techniques for broadening the area of communication of the radio-frequency tag communication system. Patent Document 1 identified below discloses an example of such techniques, that is, an interrogator system which includes a plurality of radio-frequency tag communication devices, and a control terminal for controlling the plurality of radio-frequency tag communication devices. In this interrogator system, the areas of communication of antenna devices of the radio-frequency tags overlap each other so that radio communication can be effected by the same signal between the plurality of radio-frequency tag communication devices and the radio-frequency tags, whereby the area of communication of the radio-frequency tag communication system is broadened.
In one proposed form of the radio-frequency tag communication system, the radio-frequency tags located in a given area are detected by coordinated operations of the plurality of radio-frequency tag communication devices. Patent Document 2 identified below discloses an example of such form of the radio-frequency tag communication system, which includes a plurality of fixed terminals (stationary radio-frequency tag communication devices) disposed in a given space such as an indoor space, and portable terminals (portable radio-frequency tag communication devices) that are carried by users. This radio-frequency tag communication system is capable of adequate detection of a desired destination or adequate navigation in a relatively narrow area such as an indoor space.
Patent Document 1: JP-2003-283367 A
Patent Document 2: JP-2001-116583 A
A so-called “passive tag” not incorporating a power supply source is generally incapable of data communication when the passive tag receives a plurality of transmitted signals at one time. Where the plurality of radio-frequency tag communication devices are spaced apart from each other by a distance large enough to prevent the passive tag from concurrently receiving the signals transmitted from the radio-frequency tag communication devices, the passive tag may fail to receive from the radio-frequency communication devices an electric energy necessary for data communication with the communication devices, depending upon a specific location of the passive tag. In the area of overlapping of the areas of communication of the antenna devices of the plurality of radio-frequency tag communication devices, the passive tag cannot normally receive signals from the radio-frequency communication devices, giving rise to a risk of failure to effect adequate data communication with the communication devices. Therefore, the radio-frequency tag communication devices must be positioned relative to each other such that the areas of communication of the antenna devices overlap each other to a suitable extent, and the adjacent communication devices must be controlled to transmit the transmitted signals at different points of time. In this case, the time interval of transmission of the transmitted signal from each radio-frequency tag communication device must be sufficiently long, where the number of the neighboring or adjacent communication devices and the points of time of transmission of the transmitted signals from those neighboring communication devices are not known to each communication device. However, the sufficiently log time interval results in an increased time required for the data communication, and therefore it is difficult to optimize the time interval. Thus, there has been a need of developing a radio-frequency tag communication device and a radio-frequency tag communication system, which permit mist efficient coordinated operations of the radio-frequency tag communication devices, while preventing overlapping of communication.
The prior art described above suffers from a risk of mixing of the interrogating waves as received by the radio-frequency tag, when the interrogating waves are concurrently transmitted from the plurality of radio-frequency tag communication devices. Further, the radio-frequency tag communication device suffers from difficulty in receiving a reply wave transmitted from the radio-frequency tag in response to the interrogating wave transmitted from the communication device, which difficulty arises from the interrogating waves received from the neighboring or adjacent communication devices. To avoid these drawbacks, it is considered to control the operations of the plurality of radio-frequency tag communication devices to transmit the interrogating waves, in a time-sharing fashion. However, this control requires communication among the plurality of radio-frequency tag communication devices, for the coordinated operations of the communication devices, and undesirably considerably increases the time required for communication with the radio-frequency tags. Thus, there has been a need of developing a technique which enables the radio-frequency tag communication system to achieve efficient communication of the plurality of radio-frequency tag communication devices with the radio-frequency tags with a simple control.